The present invention relates to a method and apparatus for driving the switches of a pulse inverter, and more particularly, for forming pulse-width-modulated driving voltages for driving the switches of a pulse inverter having a plurality of outputs, and, in particular, a transistor inverter.
In a pulse inverter, each output is connected alternatingly via a pair of switches to the two poles of a d-c input voltage, where the ratio of the respective switching times determines an average value for the output voltage at the inverter output.
A reference waveform of predetermined shape can thereby be obtained as the respective output voltage by controlling the closed times of the one switch or the open times of the other switch in accordance with the pulse durations of a pulsating switch control voltage, i.e. a reference voltage which is pulse-width-modulated by a high-frequency clock frequency and is proportional to the desired output waveform.
Generally, a symmetrical system of output voltages is required for driving the different outputs of the inverter. In particular, differing waveshapes between outputs or d-c components are to be avoided. It is customary to form the switching pulses for the respective switches coupled to the inverter outputs by pulse-width modulation of a separate reference voltage associated with each switch. Thus a system of reference voltages, wherein the reference voltages are as strictly symmetrical as possible, is required, i.e., the reference voltages must be distributed equidistantly in accordance with the number of phases of the inverter, and have the same waveshape.
If, for instance, thyristor switches are used as the inverter switches, certain protection times must be observed between the closing of the one and the opening of the other switch. This limits the magnitude of the sampling frequency, and the derivation of the switching commands to the switches from the driving voltages formed by pulse width modulation requires additional measures. This is eliminated if switching transistors are used as inverter switches, which can be switched at high clock frequencies (for instance, 1 to 6 kHz). With switching frequencies of such magnitude, however, the symmetry of the driving voltages must be maintained particularly strictly because small asymmetries can lead to large d-c components. Therefore, unequally fast transitions between the "1" state and the "0" state of the comparators used for the pulse width modulation of the reference voltages and other component tolerances must be carefully matched to each other, which necessitates, as a rule, time-consuming adjustment of, e.g., 6 or more trimmers.
For the loads supplied by the inverter, sinusoidal currents are usually desired. It is known that these sinusoidal currents can frequently be generated, for instance, in the case of 3-phase inverters, with particularly good utilization of the inverter, if output voltages with trapezoidal waveform are generated. In this case, pulse width-modulated symmetrical trapezoidal voltages must therefore be generated as the driving voltages for the inverter switches.